Semiconductor power converters, in the form of power controllers, for example, have fulfilled a key function in power electronics for years. Their field of application covers all power classes. In motors, mechanical commutators were replaced by electronic circuits, transformers became unnecessary in many applications, or components of the same size exceeded the power flow they provided by orders of magnitude (by generating higher frequencies). Many different applications only became feasible or financially viable with the advent of electronic conversion. This was true of distributed feed to the local grid with photovoltaic systems as well as DC power transmission and coupling with AC voltage networks with higher power flow (without the use of mechanical motor-generator sets).
Even so, these systems are still hindered by a number of problems that have yet to be solved satisfactorily. Linear controllers are not usable even with medium power flows because of their substantial energy losses. On the other hand, the use of clocked circuit diagrams in conjunction with current- or voltage-limiting components with first order dynamics causes the generation of huge harmonic waves and not only simultaneous emission by precisely said components, but also oscillations that propagate on lines themselves. In particular, various power semiconductors, such as SCR and GTO, become very unstable in response to fast voltage dynamics. Commutation problems in hub converters or breakover firing in thyristors demand extremely sophisticated filter equipment, which typically work with the conversion of higher harmonics into heat.
In addition, power semiconductors can only be manufactured and used practically for relatively low voltages of a few kilovolts. Where voltage, output, harmonic components, distortion and particularly failure safety must satisfy stricter requirements, the focus is trained almost exclusively on multilevel converter systems. The generally known modular variant, the modular multilevel converter as described in greater detail in DE 102 17 889 A1 is unrivalled in this field.
This converter system is able to convert practically any time-related voltage curves from the terminal pairs on one side into equally diverse profiles between the terminal pairs on the other side, without having to differentiate between an input and an output based on its working principle. With this modular multilevel converter, each phase of the converter is constructed from a multiplicity of identical single modules connected in series. Each individual module functions as a two-terminal network and contains an energy storage element in the form of a capacitor, and a plurality of switching elements that can optionally absorb or emit power for both voltage directions, and can thus reach all four quadrants of the current-voltage graph. In particular, these individual modules can be switched to the following four states via their switches:                Specification of a positive terminal voltage with any current direction;        Specification of a negative voltage with any current direction;        Bypass state (that is to say no energy absorption or emission by the individual module), free current flow in any direction;        Forced energy absorption by the individual module by imposing the voltage level.        
Such an individual module is thus—with corresponding control for example with clocked switching of the active elements (perhaps analogously with pulse width modulation)—already capable of controlling its own energy absorption and emission without restriction and approximately simulating a virtual load with certain properties to a source. These modules can now be interconnected for full four-quadrant operation for n sources (for example two incoming voltage systems) and m outputs (for example a three-phase low-voltage system) depending on the desired application.
A combination of two series connections, each consisting of z modules is called a phase module, wherein each of the two series connections forms a bridge branch. The number z of modules in each bridge branch defines voltage and harmonic properties of the converter. The phase modules in turn form the base modules of a single- or multiphase converter. Thus, for example, with two interconnected phase modules one system can be used to convert a 1-phase AC voltage or DC voltage to another 1-phase AC voltage or DC voltage. Such a system is configured entirely symmetrically with regard to inputs and outputs, and thus enables full four-quadrant operation with respect to each connection pair. Moreover, the behaviour of the current converter in terms of inductive or capacitive action can be adapted individually at both the input and the output sides. Consequently, energy can also flow in both directions, and can be changed dynamically.
Additionally, a system for converting a 3-phase AC voltage to a 1-phase AC voltage or DC voltage, for example, can be created with three interconnected phase modules. The combined connections of the phase modules may also be thought of as a (DC voltage) conductor rail, so that the interconnection of n+m phase modules creates a grid link for connecting an n-phase grid to an m-phase grid.
Despite the enormous advantages of the aforegoing, including—with no claim as to completeness—excellent failure safety due to the integrated redundancy configuration, automatic voltage symmetrisation of modules and semiconductors theoretically already in place, full four-quadrant operation, firmly defined maximum required blocking voltages for all semiconductors well below the maximum voltage of the terminal pairs for the overall system, even this topology still has a few problems that are not satisfactorily resolved. Some of these weaknesses, such as the immense design effort required, will be overcome with the art described in patent application DE 10 2010 052 934.6, which has not been published at the time of filing of the present application. The improvement described here already offered the ability to use the new modules and their storage capacities more effectively by virtue of the additional switching states, and possibly even to enable a charge balance between the modules. However, this last was not a primary objective.
Said currently unpublished application DE 10 2010 052 934.6 describes a special variant of the modular multilevel converter, which is also constructed from multiple individual modules, but is also designed so that the capacitors used as energy storage elements can be connected either in parallel or in series. In this context, the individual modules are arranged in such manner that the energy storage elements can be connected either in parallel or in series via internal circuit elements, so that no additional external switches are needed, as for matrix addressing.
At the same time, this described art ensures that the voltage load of the internal switching elements is not significantly greater than the maximum voltage of the capacitors.
The fundamental advantage of such an optional interconnection consists in that parallel connection of the energy storage elements in the individual modules can reduce the total internal resistance of the converter (or of a branch of the converter), so that the converter in this switching state is able to provide several times more power than is possible for conventional converters with low output voltages. This also enables the size of the capacitors in the individual modules to be reduced, depending on the use case. Moreover, such a system can also increase DC- and AC voltages extremely efficiently from the source to the load in converter mode, if many module capacitors are connected in parallel for taking up energy from the source, and if the same capacitors are then correspondingly connected in series for delivering the energy.
However, in this circuit the problem of a lossy charge balance may occur. In this converter variant, individual module capacitors should preferably only be switched from serial mode to parallel mode if the individual voltages of the capacitors that are to be connected in parallel are the same. Otherwise, charge balancing will take place, involving energy losses as illustrated in the following example:
It is assumed that modules have two capacitors 1 and 2, wherein the first capacitor has capacitance C1 and voltage U1, a second capacitor has capacitance C2 and voltage U2.
When both modules, or the capacitors thereof, are connected in parallel, a new capacitance is obtained: Ctot=C1+C2.
At the same time, however, charge Qtot, new of the total system must be maintained, so that the new charge is obtained from the total of the two partial charges of capacitors C1 and C2:Qtot,new=Q1+Q2=C1·U1+C2·U2 
This total charge can be used on the one hand to calculate the voltage of the system with capacitors connected in parallel, by the
            U      new        =                            Q                      tot            ,            new                                    C          tot                    =                                                  C              1                        ·                          U              1                                +                                    C              2                        ·                          U              2                                                            C            1                    +                      C            2                                ;and on the other hand the energy stored Etot in the system with capacitors connected in parallel can be calculated with:
      E    tot    =                    1        2            ⁢                        C          tot                ·                  U          tot          2                      =                  1        2            ⁢                        (                                                    C                1                            ·                              U                1                                      +                                          C                2                            ·                              U                2                                              )                                      C            1                    +                      C            2                              
If U1≠U2, this energy is less than the energy that was stored previously in the individual capacitors:
      E    tot    ≤                    1        2            ⁢                        C          1                ·                  U          1          2                      +                  1        2            ⁢                        C          2                ·                  U          2          2                    
The difference in energy between this and the energy that was stored in the capacitors previously is calculated with
            Δ      ⁢                          ⁢      E        =                            1          2                ·                                            C              1                        ·                          C              2                                                          C              1                        +                          C              2                                      ·        Δ            ⁢                          ⁢              U        2              ,where ΔU represents the difference between the two capacitor voltages. This energy difference is lost during the parallel switching process and is dependent on the square of the voltage difference between the capacitors.
For example, if the capacitors are connected in parallel via a resistor, this energy difference in the resistor is converted into heat. Since the energy difference is not dependent on the value of the resistor, only the current, the output and the charge reversal time are changed by different resistor values.
Specifically in the converter circuits described, however, a parallel connection operation of such kind would have to be performed with very small (protective) resistors, because otherwise the resistors—which are permanently located in the current path of the converter—lead to significant losses by the current converter. This in turn creates high charge reversal currents, which can place a heavy load on the semiconductor switch of the converter.
For a system consisting of n modules to be connected in parallel, a voltage obtained correspondingly after the parallel connection is calculated as follows:
      U    new    =                    ∑        i            ⁢                        C          i                ·                  U          i                                    ∑        i            ⁢              C        i            
The energy difference ΔEn between the sum of the individual capacitor energies and the energy of the system connected in parallel is calculated by:
      Δ    ⁢                  ⁢          E      n        =            1      2        ⁢          (                                    ∑            i                    ⁢                                    C              i                        ·                          U              i              2                                      -                                            (                                                ∑                  i                                ⁢                                                      C                    i                                    ·                                      U                    i                                                              )                        2                                              ∑              i                        ⁢                          C              i                                          )      
In converter according to the art described in application DE 10 2010 052 934.6, which was not published at the time the present application was drafted, it must therefore be ensured that the voltage differences between capacitors to be connected in parallel never become too great during operation, or if they do, they must only connected in parallel very infrequently. Otherwise relatively large energy losses and large balancing currents will occur in the individual modules of the converter.
On the other hand, if such a modular converter is to be used as widely as possible, it will almost inevitably entail voltage differences between the module capacitors, since on the one hand it is practically impossible to ensure that the same charge is applied to or drawn from all capacitors involved using one converter controller; furthermore, even the—occasionally substantial—capacitance tolerances of their module capacitors result in corresponding voltage differences.
Precisely in systems with large module voltages and fast clock speeds and/or frequent changing of the module capacitors from serial to parallel connection, this causes unnecessarily high charge reversal losses.